This invention relates to methods for transferring heat between process liquor streams, to effect simultaneous heating of one stream and cooling of another, while maintaining both of the streams in liquid phase. As used herein, the term "process liquor stream" refers to a flow of liquid (which may or may not contain solid particulate material, e.g. as a slurry) in an industrial or like process, under transport between successive process operations performed on or with the stream at respectively different temperatures. In an important particular sense, to which detailed reference will be made below for purposes of specific illustration, the invention is directed to methods for transferring heat between process liquor streams in the Bayer process for producing aluminum oxide from bauxite.
The need to transfer heat to or from liquors and/or slurries with a tendency to scale occurs frequently in the chemical and metallurgical industries, as, for example, in hydrometallurgical operations for the extraction of metal values from solid ores into alkaline or acidic extracting solutions. In many cases, as in the digestion of bauxite in the Bayer process, it is necessary to raise the temperature of the slurry to some extraction temperature well above atmospheric boiling point, and then cool the digested slurry back to approximately atmospheric boiling point before the insoluble solid residues can be separated.
In the Bayer process, bauxite ore is mixed with caustic liquor and treated ("digested") under conditions of elevated temperature and pressure to dissolve the aluminum oxide values in the ore as sodium aluminate. After digestion, the hot liquor or slurry, carrying insoluble ore material ("red mud") and dissolved sodium aluminate, is cooled, and the red mud is separated from the liquid of the slurry: the liquid, containing the dissolved sodium aluminate, commonly referred to as pregnant liiquor, is then cooled further and treated (as by seeding with aluminum hydroxide and stirring) to precipitate aluminum hydroxide. The coarsest part of this precipitate is separated out and ultimately calcined to obtain alumina as the end product, and the finer part is redirected as seed, while the remaining liquid ("spent liquor") is recycled, possibly evaporated and regenerated with addition of caustic, and heated to treat fresh quantities of bauxite.
To save energy costs, it is advantageous to effect as much as possible of the reheating of the spent and/or regenerated liquor by heat transfer from the hot digested slurry, which is to be cooled. That is to say, it is highly desirable to be able to recover the sensible heat from the digested slurry stream being cooled into the spent and/or regenerated liquor stream being heated as completely as possible so as to economize to the greatest possible extent on the quantity of heat which has to be supplied from an external source (almost always derived from burning a relatively expensive fuel).
Theoretically, the heat exchange might be effected through a heat-exchange wall such as a heat-conductive tube or plate having opposite surfaces respectively in contact with countercurrent flows of the digested slurry and spent/regenerated liquor streams. Problems of scale formation, however, have prevented successful use of such arrangements. Scale from the caustic slurry and spent/regenerated liquor streams deposits on both surfaces of the heat exchange structure, decreasing its thermal transmissivity; and while the scale can be removed from one such surface (especially, from the inside of a cylindrical conduit), removal of scale from the opposite surface, in a heat exchanger of otherwise practicable design, is extremely difficult and inconvenient.
Thus, while it would be very desirable to accomplish the heat exchange by slurry-slurry or slurry-liquor heat transfer, there is a real problem of finding suitable equipment in which to accomplish this, because of the well-known tendency of these streams to produce tenacious heat-insulating scales on heating surfaces, which must be cleaned periodically; the problem is especially severe for operation in the temperature ranges in which scales (such as calcium titanates) are formed which are difficult or impossible to remove in practice by chemical means, so that mechanical or high-pressure liquid jet cleaning has to be used. In the case of shell and tube exchangers, scale could form on both sides of the tubes; while the inside surfaces of the tubes could be cleaned mechanically or by jetting (hydraulically) as they are now, the outside surfaces are largely inaccessible even in heaters with removable bundles. Plate heat exchangers, for which all surfaces are readily accessible, might seem to answer the need, but in fact plate heat exchangers are applicable over only a limited range of pressures due to gasket problems, especially in the larger sizes, and the close clearances involved between the plates and at the feed and offtake points make their use problematic in the case of abrasive slurries.
At present, most Bayer plant circuits accomplish transfer of heat between the caustic process liquor streams by carrying out the cooling of the digested slurry (which is initially at elevated pressure) by flashing the slurry stepwise, at gradually decreasing pressures, collecting the steam flashed at each pressure level, and condensing the steam on the shell side of shell and tube heat exchangers (or on the jacket side of concentric tube digesters) for heating the liquor or slurry passing to or through the digesters. After separating the red mud residue, further heat exchange is carried out between the pregnant and spent liquor streams to cool the former to the temperature required for precipitation, and typically this is effected by a comparable flashing/heat exchanger system operating at below atmospheric pressure ("vacuum heat exchanger"). This procedure has two main advantages: first, provided that carryover of spray droplets with the flashed steam is eliminated or avoided by suitable design of the flash tanks and mist separators, the shell side of the heat exchanger or the jacket side of the tube digester is in contact only with condensing steam, and apart from impurities dissolved in the steam (e.g. steam-distilled complexes) there is relatively little danger of formation of scale on this side of the heat transfer surface; and second, the steam which is condensed in the heat exchanger shells or the tube digester jackets is removed from the system, thereby aiding the water balance of the process by providing evaporation.
However, the flashing method of heat recovery suffers from significant thermal disadvantages. Because of the nature of the flashing process, and because the boiling point of the aqueous caustic liquor is elevated above that of water, the recovered heat is degraded by the boiling point elevation of the solution being flashed, which may be of the order of 15.degree. C. (above that of water) when high caustic concentrations are used in digestion, and even at relatively low caustic concentrations in digestion is still of the order of 5.degree. C. Hence, even assuming no fouling of the heat transfer surfaces and a very large heat transfer area per heater and very many stages (i.e., even assuming the limiting case of zero approach temperature difference in the exchanger and an infinite number of stages) the minimum temperature that the digested slurry can achieve in digestion systems fed with liquors with high caustic concentrations, will be of the order of 15.degree. C. higher than if the heat had been recovered between the streams by means of slurry-liquor or slurry-slurry surface heat exchange, with comparable fouling and heat transfer area. In addition, since it is impractical to have an infinite number of heat recovery stages, in industrial practice there is a finite temperature drop (typically of the order of 10.degree. C.) as the slurry being cooled passes from one stage of flashing to the next. Thus, the temperature of the cooled digester slurry achieved in industrial practice using a heat recovery system of flashing stages in series is up to about 25.degree. C. hotter in the high caustic concentration case than the thermodynamic minimum achievable in a heat recovery system of countercurrent surface heat exchangers. The heat equivalent of this unrecovered 25.degree. C. has to be supplied to the entire digestion flow at the maximum digester temperature (i.e., as high grade heat), at considerable additional cost for energy.
Moreover, flashing from a pregnant stream in the Bayer process concentrates the liquor (in terms of caustic and alumina concentrations, but not, unfortunately, with respect to the ratio of alumina to caustic) between digestion and precipitation, whereas to maximize the liquor productivities in digestion and in precipitation it is desirable to avoid any concentration of the liquor passing from the former to the latter. Achieving evaporation by flashing from the pregnant liquor in digestion (and in vacuum cooling) is therefore counterproductive to maximum liquor productivity in the circuit and is paid for in terms of reduced energy efficiency elsewhere in the circuit, nor is it usually possible to achieve as much evaporation per unit of energy expended in the flashing systems designed for cooling as in an evaporator (on the spent side of the circuit) which is specifically designed for evaporating. It will be apparent that in the case of a high-caustic flow system involving an evaporator on the spent side of the circuit which will normally be quite large enough to provide all the mud and hydrate washing water that can be economically justified (except possibly in the case of very poor grade bauxites with exceptionally large mud factors), the additional evaporation due to flashing in digestion and vacuum cooling has little economic value in any case.
Tube digesters are known to be useful in hydrometallurgical operations for the extraction of metal values from solid ores into alkaline or acidic extracting solutions, especially where the extraction has to be carried out at temperatures giving rise to pressures above atmospheric, as is the case in the Bayer process extraction of alumina from bauxite into caustic soda or caustic-aluminate solutions. Among the advantages of the tube digester are the close approach to plug flow, minimization of bypassing, the high degree of turbulent mixing of the solid and liquid phases, and the fact that by enclosing the tube in a second larger tube carrying a heating or cooling medium (or several parallel tubes in a common jacket), it is possible to simultaneously effect heat transfer to or from the slurry during the extraction process.
As described for example in U.S. Pat. No. 3,497,317, concentric tube-tube digesters have been proposed for the Bayer process. In such service, however, the outside surface of the inner tube scales as well as the inner surface (which can be cleaned), and since the outer surface is enclosed by the annular jacket, it is not readily accessible for cleaning by other than chemical means by the circulation from time to time of a descaling liquid of a suitable type through the jacket space. In addition, dismantling the concentric tube device is difficult once this outer surface becomes scaled. These problems (representing a specific instance of the above-described scaling difficulty associated with indirect heat exchange between caustic process liquor streams in contact with opposite surfaces of a tube or other heat-exchange wall), have prevented successful commercial Bayer process operation of tube digesters in the direct slurry-liquid or slurry/slurry heat exchange mode, although tube digesters are in use with flashed steam on the jacket side, with the disadvantages noted above.
The abovedescribed difficulties associated with heat transfer between digested slurry and spent liquor in the Bayer process exemplify some of the types of problems that can be overcome or minimized by the present invention. Other problems also overcome by the invention, in diverse environments of use, are discussed below.